The present invention relates to a magnetic recording medium having a lubricating layer, which contains an organic fluorine compound on a magnetic layer, and in particular to a magnetic recording medium, which has a magnetic layer consisting of ferromagnetic metallic thin film with higher lubricating property and rust preventive property.
Magnetic recording medium such as magnetic tape, floppy disk, etc. is used with lubricant coated on the surface of the magnetic layer to increase lubricating property and running durability between the magnetic recording medium and a head.
In particular, with rapid technical progress toward the recording with higher density in magnetic recording medium, the surface of the magnetic layer is getting increasingly smoother. Also, magnetic recording medium is used in various environmental conditions, and the recorded information is stored over a period from several years to several tens of years. For this reason, there are strong demands on the medium with high running durability and preservation stability under various environmental conditions, and sufficient results cannot be obtained by conventional type lubricant.
Further, cam-corder, i.e. camera-integrated video tape recorder or small size video tape recorder are frequently used in outdoor application, and high lubricating performance characteristics suitable for the use under extensive environmental conditions are required for a magnetic recording medium.
Metal thin film type magnetic layer is advantageous for high density recording, and there are much expectations on the metal thin film type for the improvement of magnetic recording density such as the implementation of high-television system or digitalization in future. Because metal in the recording layer is protected only by very thin oxide layer in the metal thin film type magnetic recording medium, there is imminent demand on a lubricant, which can extensively improve the preservation property.
It is known that it is effective to use an organic fluorine compound as lubricant in such metal thin film type magnetic recording medium.
It is widely known that an organic fluorine compound containing perfluoroalkyl group has hydrophobic as well as lipophobic property and has the property as surface active agent in both water and oil. Also, it has the property to decrease surface energy, and the organic fluorine compound is practically used by coating it to improve the sliding property or lubricating property of a material.
For example, Japanese Patent Laid-Open Publication No. 58-100228 discloses a technique, in which a compound having sulfonic metal salt as polar group, an ester group as connecting group, and two fluorine-substituted alkyl groups on molecular terminals is mixed in a coating solution containing dispersed magnetic powder, and this is coated. Because this compound contains sulfonic acid metal salts as polar groups and the polar groups have high polarity and have strong property as acid, and there are problems such as corrosion of metal thin film or thin film head used for high density recording. Because chain length of molecular chain of perfluoroalkyl group is short, sufficient decrease of friction coefficient cannot be expected when it is used as protective layer of ferromagnetic metallic thin film type magnetic recording medium. Further, it is inferior in solubility in non-fluorine type organic solvent, while fluorine type organic solvent must be used to form the protective layer, and this causes environmental problem. Also, the lubricant itself is susceptible to hydrolysis and cannot endure long-term storage.
Japanese Patent Publication Laid-Open 6-253634 describes a lubricant expressed by (C.sub.n F.sub.2n+1 Y.sub.m O).sub.k =O(OX).sub.3-k, and alkylene group, alkyleneoxide or sulfonamide are disclosed as connecting group for phosphoric acid ester and fluorine chain, while this is not soluble in non-fluorine type solvents.
Japanese Patent Publication 3-19602 and Japanese Patent Publication Laid-Open 54-79366 describe lubricants, comprising fluorine type hydrocarbon compound having --SO.sub.3 Na, --SO.sub.3 K, --SO.sub.3 H, --COOH, etc., and concrete examples are given such as C.sub.8 F.sub.17 SO.sub.2 F, terminal OH or SO.sub.3 H of perfluoroalkyl polyoxyethylene, or terminal betaine structure of R.sub.f CONH(CH.sub.2).sub.3, but these compounds are inferior in .mu. value, still durability and preservation property and are not soluble in hydrocarbon type solvents.
Japanese Patent Publications Laid-Open 3-241524 and 62-92227 describe coating of compounds having perfluoroalkyl group and aliphatic alkyl group on ferromagnetic metallic thin film. These compounds can be coated by means of isopropanol, while these are inferior in .mu. value, still durability and preservation property.
Japanese Patent Publication Laid-Open 64-72313 describes formation of monomolecular layer of perfluoroalkyl group - alkyl group - polar group on ferromagnetic metallic thin film, while formation of monomolecular film by Langmuir-Blodgett (LB) film is low in productivity. Further, the compound is inferior in .mu. value, still durability and preservation property.
In addition to the coating of the above lubricant on magnetic recording layer, the use of perfluoroalkyl polyether and its derivatives is described (Japanese Patent Laid-Open Publications 60-61918, 61-07528, U.S. Pat. 3,778,308, Japanese Patent Publication 60-10368, U.S. Pat. 4,897,211, etc.). However, the lubricants containing fluorine type organic compound has high friction coefficient at low speed with the partner material when tape is sliding. The lubricants have also problems with preservation property because magnetic layer is detached due to corrosion by acidic gas such as sulfur dioxide and are not suitable as magnetic recording medium used in practical application.
Fatty acids and fatty acid esters (Japanese Pat. Publications 28-28367, 51-39081, 51-30981, Japanese Patent Publications Laid-Open 56-80828 and 61-24017) have high speed friction with magnetic head and are not suitable for practical use. It is also inferior in the problem of corrosion due to acidic gas such as sulfur dioxide.
Most of the above lubricants are not soluble in organic solvents of normal hydrocarbon type, such as acetone, methylethylketone, ethyl acetate, alcohols, toluene, etc., and it is necessary to use fluorine type organic solvents when coating. However, fluorine type organic solvents are expensive and also cause the problem of the destruction of ozone layer or aggravate the working environment and are not suitable for practical application.
It is an object of the present invention to provide a lubricant, which is soluble in general hydrocarbon type organic solvents and contains fluorine type organic compound without the need of using fluorine type organic solvents and also to provide a magnetic recording medium, which comprises metal thin film for extensively improving recording density by deposition, sputtering, etc. and which can offer high running property, durability and preservation property.
Further, it is another object of the present invention to provide a magnetic recording medium which has sufficient running property and durability even in outdoor environment and which does not corrode magnetic head for high density recording such as thin film head.